The present invention relates to an automatic focusing system of a video camera or the like and, more particularly, to an automatic focusing system in which a master lens group of the lens system is finely vibrated in the optical axis direction, a high frequency component (focus voltage) is detected from a video signal obtained by an image pickup device, and a moving mechanism to move the master lens group is controlled so that a magnitude (amount) of the high frequency component becomes maximum.
Hitherto, as one of automatic focusing systems attached to video cameras and the like, there has been known a system disclosed in U.S. Pat. Ser. No. 4,611,244.
Such a conventional automatic focusing system comprises: means for generating an automatic focusing signal according to the lens position of the lens system of a video camera; a motor to move a part or all of a master lens group of the lens system; motor drive control means for performing an automatic focusing operation by controlling the driving of the motor in accordance with the automatic focusing signal; switching means which is switched between first and second modes to select either a photographing mode in an ordinary distance range or a photographing mode in a macro distance range; and master lens moving range control means for selecting a movable range of the master lens in accordance with the mode of the switching means and for controlling the motor drive means so as to move the master lens within the selected moving range.
According to this system with such a construction, the focusing operation is performed by moving the master lens and the moving range of the master lens in the photographing in the ordinary distance range and the moving range of the master lens in the photographing in the macro distance range are made different, thereby enabling the photographing in the macro distance range of the video camera to be performed. In the case of performing the focusing operation by using both of an objective lens which is fixed to a predetermined position and a master lens which is moved (deviated) by a moving mechanism, the moving mechanism can be miniaturized and reduced in weight than the case of using an objective lens which is moved by the moving mechanism. This is because the objective lens is generally larger and heavier than the master lens.
On the other hand, such a conventional automatic focusing system includes an image pickup device for converting a two-dimensional optical image formed by the lens system into a time-sequential electric signal (video signal) and a camera circuit. The high frequency component corresponding to the fineness of the optical image (image) is extracted from the video signal by a high pass filter. Further, the signal of the high frequency component is detected by a detector and converted into a focus voltage which is proportional to a magnitude (amount) of the high frequency component signal.
Therefore, the focus voltage depends on the fineness of the optical image and becomes maximum when the optical image is accurately focused. As shown in FIG. 1, assuming that an object to be photographed is located at the position which is away from the video camera by A (m), the focus voltage becomes maximum when a scale position for adjustment of the distance of the master lens exists at the position of A (m) and decreases as the scale position is deviated from A (m). As will be understood from FIG. 1, the focusing operation is automatically performed if the position of the master lens is set so that the focus voltage becomes a maximum value.
In the conventional automatic focusing system, a difference detector circuit is provided to set the focus voltage so as to become maximum. In the difference detector circuit, the focus voltage is sampled and held every predetermined time and a positive voltage is generated when the focus voltage increases with an elapse of time, while a negative voltage is generated when the focus voltage decreases with an elapse of time.
An output voltage of the difference detector circuit corresponding to the focus voltage is shown in FIG. 1 together with the focus voltage. The master lens is moved in the direction in which it is at present set when the output voltage of the difference detector circuit is positive. The master lens is stopped when the output voltage changes from the positive value to a negative value. (Practically speaking, the master lens is finely vibrated at the position near the maximum value of the focus voltage.)
The conventional automatic focusing system automatically executes the focusing operation as explained above. However, in this first conventional example, nothing is disclosed with respect to the practical moving mechanism of the master lens (although there is a disclosure of a motor) and, further, nothing is also disclosed with respect to the moving speed of the master lens.
Next, as a second conventional example, there is a system disclosed in a paper entitled "VHS Movie Cameras NV-M1 and NV-M3" disclosed in National Technical Report, Vol. 31, No. 6, pages 812 to 823, published on December, 1985.
An automatic focusing system similar to that disclosed in the foregoing U.S. Pat. Ser. No. 4,611,244 is disclosed at page 822 in that paper. For instance, FIG. 2 in this paper shows a change in high frequency component to a deviation of a focal point of an optical image which is caused due to a change in optical path length between an object to be photographed and a photo sensing surface of an image pickup device due to a micro vibration (constant amplitude) of the master lens.
As shown in FIG. 2, the micro vibration component of the high frequency component does not change at the focal position in the case where the master lens was finely vibrated, and the phases of the changes in the improper focus states of the before-focal point and after-focal point differ by 180.degree.. In this case, an amplitude of micro vibration component corresponds to a magnitude of output voltage of the difference detector circuit in the first conventional example. On the other hand, a phase of micro vibration component similarly corresponds to a polarity of output voltage of the difference detector circuit.
Therefore, in the second conventional system, the phase of the micro vibration component is detected by using the change in optical path length as a reference, the moving direction of the focusing lens is determined from the phase of the micro vibration component, a point at which the amplitude of the micro vibration component becomes zero is detected, and by using that the point of the zero-amplitude corresponds to the position of the maximum value of the high frequency component, the movement of the lens is stopped. In this conventional system, an objective lens is used as the focusing lens.
A third conventional system for performing the focusing operation by changing the optical path length and by moving the objective lens in a manner similar to the second conventional example is disclosed in JP-A-60-42723.
In the second and third conventional examples, since both of the objective lens and master lens are moved (vibrated), there are problems such that the moving mechanism is complicated and its size and weight are large. On the other hand, similarly to the first conventional example, nothing is also disclosed with respect to the moving speed of the object lens or the like.
A fourth system to improve the foregoing problems in the second and third conventional examples is also further proposed. According to the fourth system, the master lens is moved in a manner similar to the first conventional example while finely vibrating it. Thus, the moving mechanism can be simplified and the automatic focusing system can be reduced in size and weight.
The fourth conventional example is disclosed in Japanese Patent Application No. 62-24586 (corresponding to U.S. Pat. Application No. 151,963 filed on Feb. 3, 1988).
According to the fourth automatic focusing system, a focusing mechanism is provided in the rear portion of a variator lens of the zoom lens system of a video camera and this focusing mechanism is moved while it is being finely vibrated at a predetermined reference frequency. On the other hand, according to this system, the high frequency component is extracted from the video signal derived from the image pickup device and the micro fluctuation component corresponding to the micro fluctuation of the reference frequency is detected from the high frequency component. The focusing mechanism is driven by a motor in accordance with the polarity and amplitude of the micro fluctuation component detected. Thus, in a manner similarly to the case of FIG. 2, the high frequency component signal (focus voltage) has the maximum value and the focusing operation is automatically executed.
The fourth conventional system will now be described in more detail with reference to FIGS. 3 and 4
FIG. 3 is a block diagram of an embodiment of an automatic focusing system according to the fourth conventional system.
In this automatic focusing system, the master lens is moved while it is finely vibrated.
In FIG. 3, reference numeral 1 denotes a lens system of a video camera; 2 is an objective lens fixed to a predetermined position; 3 a variator lens to change a magnification of the lens system 1 in a zooming mode; 4 a compensator lens which is moved in the zooming mode and corrects a focusing deviation during the zooming operation for an object to be photographed; 5 a diaphragm apparatus; 6 a master lens; and 7 an image pickup device in which an optical image is formed on the photo sensing surface by the master lens 6.
Reference numeral 17 denotes a stepping motor which is driven by an input pulse and rotates its rotating shaft in accordance with a period and a phase of the pulse. Reference numeral 19 denotes moving means for holding the master lens 6 and moving the master lens 6 in the optical axis direction of the lens system 1 while allowing it to be finely vibrated. Reference numeral 18 indicates a gear to propagate the rotational force of the rotating shaft of the stepping motor 17 to the moving means 19. The rotation of the motor 17 is converted into the rectilinear motion in the optical axis direction of the master lens 6.
Reference numeral 12 represents a control circuit having a reference signal generating source 13, a control signal generating circuit 14, and a sync detecting circuit 15. The reference signal generating source 13 generates a reference frequency signal to finely vibrate the master lens 6 at a predetermined period. The reference frequency signal generated from the reference signal generating source 13 is supplied to the control signal generating circuit 14 and sync detecting circuit 15. The control signal generating circuit 14 generates a control signal to move the master lens 6 while finely vibrating the master lens on the basis of both of the reference frequency signal supplied from the reference signal generating source 13 and a signal supplied from the sync detecting circuit 15. This control signal is supplied to a drive circuit 16.
The drive circuit 16 drives the stepping motor 17 on the basis of the control signal supplied from the control signal generating circuit 14 in the control circuit 12. The stepping motor 17 is driven by an output of the drive circuit 16. The master lens 6 moves while finely changing the focusing state at such a fine degree that cannot be detected by the human eyes. Therefore, an output signal of the image pickup device 7 also finely changes in correspondence to the micro vibration of the master lens 6.
Reference numeral 8 denotes a preamplifier to amplify the output signal of the image pickup device 7. An output signal of the preamplifier 8 is supplied to both of a camera circuit 9 and a high frequency component extracting circuit 10. The camera circuit 9 makes a video signal on the basis of the output signal of the preamplifier 8. The camera circuit 9 outputs the video signal to the outside of the video camera main body.
The high frequency component extracting circuit 10 extracts the signal of the high frequency component from the output signal of the preamplifier 8 and outputs. The high frequency component signal includes the signal corresponding to the micro vibration of the master lens 6 since the focusing state finely changes. Reference numeral 11 denotes a detecting circuit to detect a signal of the micro vibration component from an output signal of the high frequency component extracting circuit 10. The detecting circuit 11 outputs the detected signal to the sync detecting circuit 15 in the control circuit 12.
The sync detecting circuit 15 synchronously detects an output signal of the detecting circuit 11 by using the reference signal supplied from the reference signal generating source 13. By this synchronous detection, the polarity and amplitude of the output signal of the detecting circuit 11, that is, the polarity and amplitude of the micro vibration component in the high frequency component of the video signal are detected. The detection signal of the polarity and amplitude is supplied to the control signal generating circuit 14. Thus, the control signal generating circuit 14 outputs the control signal so as to make the magnitude of the high frequency component of the image pickup device 7 maximum, thereby allowing the focusing operation to be automatically performed in a manner similar to the second conventional system.
The fourth conventional system is not limited to the case of using the stepping motor. It is preferable to use a motor such as stepping motor, ultrasonic motor, or the like which is driven by pulses.
FIG. 4 shows a conceptional diagram in the case where the master lens 6 is moved while it is finely vibrated in the fourth conventional system.
In FIG. 4, FIG. 4(1) shows a graph in which an axis of abscissa denotes a time t and an axis of ordinate indicates a position of the master lens 6. The position of the master lens 6 is indicated by values which are obtained by dividing the distance from the infinite distance .infin. to the close range into n steps. FIGS. 4(2) and 4(3) show a timing and a phase (indicated by the direction of an arrow) of an input pulse to rotate the stepping motor 17 every step in FIG. 4(1). FIG. 4(2) corresponds to the operation shown by a broken line in FIG. 4(1). FIG. 4(3) corresponds to the operation shown by a solid line in FIG. 4(1).
As will be understood from FIG. 4, the input pulses which are input to the stepping motor 17 have a predetermined period. The position of the master lens 6 is shifted by only 1/n every time the input pulse is input to the stepping motor 17. On the other hand, in the case of the solid line in FIG. 4(1), after the fine deviation of one cycle (period: T.sub.V) was performed with an elapse of the time t, a deviation is performed by one step in a period of time T.sub.M. In the case of the broken line, after the fine deviation of one cycle (period: T.sub.V ') was performed, the deviation is performed by only three steps in a period of time T.sub.M '. The micro deviation of the master lens 6 of one cycle indicates that the master lens 6 is finely vibrated. Further, a in FIG. 4 denotes the case where the number of input pulses is increased in a predetermined period.
In FIG. 4, it should be noted that the input pulses (drive pulses) to the stepping motor 17 have a predetermined period.
This means that the moving speed of the master lens 6 is constant.
A typical driving method of the stepping motor and a typical control method of the rotating speed of the stepping motor will now be described with reference to FIGS. 5 and 6.
FIG. 5 is a diagram showing a typical stepping motor and a signal of a current which is supplied to a drive circuit of this motor. In FIG. 5(1), a CLK signal consisting of a pulse train and a CW/CCW signal indicative of a forward/reverse rotation of the motor are supplied from an external circuit (not shown) to the drive circuit 16. Upon reception of these signals, the drive circuit 16 outputs signals .phi..sub.1 to .phi..sub.4 of currents of four phases to a stepping motor 17a. Upon reception of the signals .phi..sub.1 to .phi..sub.4, the stepping motor 17a rotates its rotating shaft in accordance with the states of the signals .phi..sub.1 to .phi..sub.4. The signals .phi..sub.1 to .phi..sub.4 are supplied to an exciting coil (not shown) of the stepping motor 17a.
FIG. 5(2) shows the relations among the CLK signal which is supplied to the drive circuit 16 and the signals .phi..sub.1 to .phi..sub.4 which are output from the drive circuit 16 in the case where the CW/CCW signal indicates the forward rotation (clockwise rotation) of the motor. As will be understood from FIG. 5(2), for the signals .phi..sub.1 to .phi..sub.4, the signal .phi..sub.1 is first output synchronously with the CLK signal and the other signals .phi..sub.2 to .phi..sub.4 are sequentially output in accordance with this order. The signal .phi..sub.1 is again output after the signal .phi..sub.4 was output. In this manner, when the current signals .phi..sub.1 to .phi..sub.4 are sequentially repetitively output in accordance with the order of the signals .phi..sub.1 to .phi..sub.4, the rotating shaft of the stepping motor 17a rotates forwardly. On the contrary, FIG. 5(3) shows the relations among the CLK signal and the signals .phi..sub.1 to .phi..sub.4 in the case where the CW/CCW signal indicates the reverse rotation (counterclockwise rotation). The signals .phi..sub.1 to .phi..sub.4 are repetitively output synchronously with the CLK signal in accordance with the order from the signal .phi..sub.4 to the signal .phi..sub.1. When the signals .phi..sub.1 to .phi..sub.4 are output in this manner, the rotating shaft of the stepping motor 17a rotates reversely.
FIG. 6 is a diagram showing a typical speed control method of a rotating speed in the case of driving the stepping motor. The speed control method shown in FIG. 6(1) is called a trapezoidal control system. The trapezoidal control system is used to prevent that when the drive circuit 16 outputs the signals .phi..sub.1 to .phi..sub.4 to rotate the rotating shaft of the stepping motor 17a, a rotor (not shown) connected to the rotating shaft cannot follow a change in supply of the signals .phi..sub.1 to .phi..sub.4. A phenomenon such that the rotating shaft does not normally rotate because the rotor cannot follow due to the inertia is called an out-of-step phenomenon. On the other hand, the trapezoidal control system is used to prevent that when the stepping motor 17a stops the rotation of the rotating shaft, the unnecessary vibration (ringing) of the rotating shaft based on the inertia of the rotor or the like occurs. The trapezoidal control system is used when the position to stop the rotating shaft is predetermined.
In FIG. 6(1), an axis of abscissa denotes a time t and an axis of ordinate indicates a rotating speed of the rotating shaft of the stepping motor 17a. As shown in the diagram, after the rotation of the rotating shaft of the stepping motor 17a was started, the rotating speed of the rotating shaft is accelerated for only a predetermined time. Thereafter, the rotating speed is set to the uniform speed. After an elapse of a predetermined time, the rotating speed of the rotating shaft is decelerated and the rotating shaft of the stepping motor 17a soon stops. This speed control method is called a trapezoidal control system since a change state of the rotating speed of the rotating shaft of the stepping motor 17a shows a trapezoidal shape.
FIG. 6(2) is a diagram showing the signals .phi..sub.1 to .phi..sub.4 corresponding to FIG. 6(1) and relates to the case where the rotating shaft rotates forwardly. In FIG. 6(2), when the pulse is set in the rising state, a current flows through each exciting coil of the stepping motor 17a for only the time of the pulse width. When the rotating speed of the rotating shaft of the stepping motor 17a is accelerated, the interval of each pulse gradually decreases. When the rotating speed is set to the uniform speed, the interval of each pulse is set to a predetermined time. Further, when the rotating speed is decelerated, the interval of each pulse gradually increases. FIG. 6(3) is a diagram showing the CLK signal which is supplied to the drive circuit 16 and corresponds to FIG. 6(2). When the pulse interval of the CLK signal changes from a long interval to a short interval, the rotating speed of the rotating shaft of the stepping motor 17a is accelerated. When the pulse interval is constant, the rotating speed is also constant. On the contrary, when the pulse interval of the CLK signal gradually decreases, the rotating speed is decelerated.
In the conventional automatic focusing system shown in FIGS. 3 and 4, as will be understood from FIGS. 4(2) and 4(3), the pulse interval of the input pulse (CLK signal) which is supplied to the stepping motor 17 is constant. This means that the moving speed of the master lens 6 is set to be constant. On the other hand, the moving direction of the master lens 6 rapidly changes in the opposite direction. That is, in the conventional automatic focusing system, nothing is considered with respect to the out-of-step phenomenon which is caused by suddenly rotating the rotating shaft of the stepping motor 17 at a predetermined rotating speed from the stop state. In addition, no consideration is made with regard to the unnecessary vibration (ringing) which is caused by suddenly setting the rotating shaft from a predetermined rotating speed state to the stop state (or reverse state).
When the out-of-step phenomenon occurs, the rotating shaft of the stepping motor 17 does not normally rotate. Therefore, the master lens 6 which is moved by the rotation of the rotating shaft does not normally move. Thus, it is difficult to move the master lens 6 to the position corresponding to the maximum value of the high frequency component signal of the video signal. That is, the conventional system has a problem such that the accurate focusing operation cannot be performed.
On the other hand, in the conventional system, since the master lens 6 is finely vibrated with the unnecessary vibration (ringing) caused, a predetermined micro vibration cannot be correctly executed. Therefore, the detecting circuit 11 in FIG. 3 cannot normally detect the micro vibration component included in the high frequency component signal. Thus, the sync detecting circuit 15 in FIG. 3 can hardly accurately output the detection signals of the polarity and amplitude which are necessary for the focusing operation. Namely, there is a problem such that the conventional system cannot perform the accurate discrimination of the focus.
Moreover, since the master lens 6 is moved at a uniform speed even in the portion of a large inclination of the high frequency component signal of the video signal mentioned above, it takes an extra long time until the correct focus is obtained.